Question

i.Explain why sodium is less reactive than potassium.
ii.What is meant by the inert pair effect?
iii.Write down the Aufbau’s principle.

Answer 1

The reactivity of the metals is dependent upon their electronic configuration and their position in the periodic table. If an element is placed lower in the group then it is much more reactive than the elements present early in the group. Aufbau’s principle explains the filling of orbitals.

Complete answer:
i.The attraction between the electrons and the nucleus will be higher for sodium as compared to that of potassium. Hence the valence shell electrons of potassium are looser as compared to sodium and hence are lost easier in comparison to that of potassium.
ii.The inert pair effect – the inner electrons present in the inner shells of the atoms exert a repulsive force on the outer electrons and prevent them from getting the attractive force of the nucleus. This is termed as the “shielding effect” and this is more for the s-subshell than the d-subshell. The 6s valance shell of the elements of group 13, 14 and 15 exert the shielding effect due to the presence of the 5d orbitals prior to the 6s shell which is very large and hence does not get the attraction by the nucleus properly. Due to this the 6s electrons of these elements do not get unpaired and hence do not participate in bonding. This is called the “inert pair effect”.
iii.Aufbau’s principle – The principle states that in the ground state of an atom or an ion, electrons fill orbitals of the lowest energy level first and then occupy the orbitals of higher energy.

Note:
The atomic number of sodium is 11 and that of potassium is 19, the electronic configuration of sodium is ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{1}}}$ and that for potassium is ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^2}{\text{3}}{{\text{p}}^{\text{6}}}{\text{4}}{{\text{s}}^{\text{1}}}$. So the valence shell for sodium is 3s while that for potassium is 4s.
The word “aufbau” is taken from a Greek word which means “building up”.